1. Field of the Invention
The present invention relates to blocked polyisocyanates having a built-in HALS stabilizer, a process for their preparation, and a process for producing polyurethane (PUR) powder coatings.
2. Discussion of the Background
Blocked polyisocyanates are used for producing heat-curable one pack PUR baking systems which are storage stable. The masking or blocking of polyisocyanates is a well known way of affording temporary protection for isocyanate groups. The most common blocking agent used is .epsilon.-caprolactam, which forms with isocyanates a stable compound up to about 130-140.degree. C. and which unblocks the blocked NCO groups at baking temperatures of 180.degree. C. or higher.
The isocyanates preferred for heat-curable pulverulent compositions are (cyclo)aliphatic diisocyanates because of their excellent aging characteristics compared with aromatic isocyanates, which have the disadvantage of yellowing on baking, and aging in particular.
.epsilon.-Caprolactam-blocked isocyanate-polyol adducts based on isophorone diisocyanate (IPDI) in particular have become established as PUR powder curing agents. The reason for this is very probably the difference in reactivity between the two NCO groups in the IPDI, which permits controlled adduct formation from IPDI and the polyol (NCO:OH=2:1 ) with a narrow molecular weight distribution. A narrow molecular weight distribution of the curing agent is a prerequisite for good flow of the cured powder.
.epsilon.-Caprolactam-blocked IPDI melts at 53-55.degree. C. Owing to the low melting temperature, the powders produced from this blocked IPDI cake together in storage. To increase the melting point, IPDI is subjected to a chain-lengthening reaction with polyol (NCO:OH=2:1 ) before .epsilon.-caprolactam blocking. DE-A-21 05 777 mentions polyols such as trimethylolpropane, trimethyl-1,6-hexanediol and diethylene glycol as chain lengtheners for IPDI and DE-A-25 42 191 mentions mixtures of di- and trifunctional polyols. Both are incorporated herein by reference.
The PUR powder coatings p with these curing agents have to be stabilized against degradation by radiation. The stabilizers used are the known UV stabilizers based on benzotriazole (e.g. TINUVIN.RTM. 326) or based on strongly sterically hindered anrines (e.g. TINUVIN.RTM. 770). The disadvantage with PUR powder coatings stabilized with these stabilizers is the limited lifetime of the stabilizers, which migrate to the surface over time and are destroyed there.